This is both a terrible and wonderful dilemma to have! Two interesting novel ideas at once? Yes, please.

Like others have said, I'd suggest doing the one that has your attention right now. You don't even have to Prep the one you'll use, really.

I have participated and completed NaNo in the past. I came up with the idea I wanted to use a couple days in advance by pulling a few scattered sentences and occurrences from my old idea folder, smashing them into one, and liberally sprinkling the story with every single thing I love to see in fiction. It was a self-serving mess, but it was fantastically enjoyable to write. Winning NaNo (for me), is less about preparation and more about loving what I'm doing and looking forward to the end. If I'm not excited about it, I can't write it. So my advice is to go with the one that you can't wait to write.

I wish you the best of luck in choosing whichever story you want to write. I'm sure either one will be excellent. No need to worry about preparing ahead of time. Sometimes there's value in flying by the seat of your pants (I have no idea where that idiom came from, but I like it. Another!)

All the best!